The present invention relates to an optical fiber cable to be used as a transmission medium for optical communication in communication and measurement, and also as a transmission medium for optical energy in product fabrication, medical purpose, etc.
In recent years, the use of a light and a laser beam as a communication medium and as an energy medium has been increasingly spread, and rapid progress has been made in the development of an optical fiber cable as a transmission medium to guide the light and the laser beam.
Referring to FIG. 5 and FIG. 6, description will be given on general structure of the optical fiber cable.
An optical fiber cable 1 comprises a cable main unit 2 and cable terminal units 7 and 7′ to be attached on end portions of the cable main unit 2. The cable main unit 2 comprises an optical fiber 3 and a sheath 4 to cover the optical fiber 3 and to protect the optical fiber 3. The optical fiber 3 further comprises a core 5 with high refractive index at its center and a clad 6 with low refractive index to enclose the circumference of the core 5. As the material of the core 5 and the clad 6, silica glass (quartz glass) is primarily used.
As shown in FIG. 7, when a light enters an end surface of the core 5 at an incident angle φ, the incident angle at boundary surface between the core 5 and the clad 6 is increased to a value bigger than a critical angle, which is determined by a difference between the refractive indices of the core and the clad. Then, the light is reflected in total reflection by the boundary surface, and the light propagates inside the core 5.
The cable terminal units 7 and 7′ are attached on end portions of the cable main unit 2. The optical fibers 1 are connected with each other by connecting the cable terminal unit 7 or the cable terminal unit 7′ to a connector (not shown). In case of parallel luminous fluxes, a collimator lens is used as the cable terminal unit 7′. In case of converged luminous fluxes, a parallel plate is used as the cable terminal unit 7. When the optical fiber cable 1 is connected to a distributor, an amplifier, etc. (not shown), the cable terminal units 7 and 7′ are connected to a distributor, an amplifier, etc. respectively.
Now, description will be given on the cable terminal units 7 and 7′.
The sheath 4 is removed for a certain required length at a forward end of the cable main unit 2. The optical fiber is exposed, and a ferrule 8 is attached on the exposed portion of the optical fiber 3. With the ferrule 8 attached on the optical fiber 3, an end surface of the optical fiber 3 is polished together with the ferrule 8. After polishing, an antireflection film (not shown) is formed on the end surface. The end surface of the optical fiber 3 is polished integrally with the ferrule 8 for the purpose of compensating the insufficient strength of the optical fiber 3 by the ferrule 8.
A terminal unit cover 9 is attached on the ferrule 8. On a forward end of the terminal unit cover 9, a cover glass 11 is mounted, and a gap 12 is provided as required between the cover glass 11 and the forward end of the optical fiber 3. An antireflection film (not shown) is formed on both of the end surfaces of the cover glass 11.
On the cable terminal unit 7′, a collimator lens 13, which has an end surface designed as a convex surface and an effect of a lens, is used instead of the cover glass 11. On both of the end surfaces of the collimator lens 13, antireflection films are formed respectively. When the collimator lens 13 is provided, the light projected from the cable main unit 2 is turned to parallel luminous fluxes. Or, the light transferred from the collimator lens 13 is converged to the core 5 of the cable main unit 2.
In the conventional type optical fiber cable as described above, when the cable terminal units 7 and 7′ are mounted on the cable main unit 2, the end surfaces must be polished in order to reduce optical loss such as the loss caused by irregular reflection on the cable main unit 2. Further, for the purpose of reducing the optical loss caused by the reflection on the end surfaces, the antireflection film must be provided. Also, the antireflection film is required also for the purpose of reducing the optical loss caused by reflection on the end surfaces of the cover glass 11 and the collimator lens 13.
In the treatment to fabricate the end portion of the cable main unit 2 in the conventional type optical fiber cable as described above, the end surface of the cable main unit 2 must be polished and the antireflection film must be prepared to mount the cable terminal unit 7 on the cable main unit 2. Also, the antireflection film must be prepared on the cover glass 11 and the collimator lens 13. Thus, many processes are needed for the treatment, and much time and cost are required.
When the collimator lens 13 is arranged on the cable terminal unit 7′, in order to turn the light projected from the collimator lens 13 to parallel luminous fluxes, there must be strict positional relation in a direction of the optical axis between the end surface of the cable main unit 2 and the collimator lens 13, and accurate positioning must be performed between the end surface of the cable main unit 2 and the collimator lens 13 (i.e. adjustment of opening dimension of the gap 12). For this reason, troublesome and burdensome procedure is required for maintaining accurate dimension of the gap 12 formed between the ferrule 8 and the collimator lens 13 and for mounting the terminal unit cover 9.